(1) Field of the Invention
The present invention generally relates to an amplifier, and more particularly to an amplifier having a field-effect transistor (FET) with a gate electrode biased at a potential lower than a potential of a source electrode, wherein an input signal to the gate electrode is amplified and an output signal indicating the amplified input signal is output from a drain electrode.
(2) Description of the Related Art
FIG. 1 shows a conventional amplifier having a field-effect transistor (FET). This amplifier is used as a first-stage amplifier which is provided within a low-noise amplifying unit of a global positioning system (GPS) receiving subsystem.
A global positioning system GPS is widely used for precise radio navigation. In the GPS, a network of various GPS satellites and a dozen ground stations are employed for monitoring and control positioning of a vehicle is achieved by utilizing microwave communications between the satellites and the ground stations. The vehicle derives the three-dimensional position and velocity of the vehicle from ranging signals at 1.57542 GHz received from four or more GPS satellites. The most recent position of the vehicle can be accurately indicated by longitude, latitude, and attitude by utilizing the received signals by the GPS.
A GPS receiving subsystem comprises an antenna unit, a receiver unit, a signal processing unit, and a display unit. The antenna unit includes a plane antenna element and a low-noise amplifying unit.
The low-noise amplifying unit mentioned above includes the first-stage amplifier shown in FIG. 1, a low-pass filter, and a final-stage amplifier. The processed signals whose voltage is amplified at a predetermined level and have a wavelength is restricted below the upper limit of the low-pass filter. The amplified are transmitted from the final-stage amplifier to the receiver unit in the GPS receiving subsystem.
In the GPS receiving subsystem described above, very small signals at 1.57542 GHz from the GPS satellites are received at the antenna unit, and such signals from the antenna unit are amplified by the first-stage amplifier. For this reason, a first-stage amplifier must have the high signal-to-noise (S/N) ratio. In order to meet this requirement, a HEMT (high electron mobility transistor) GaAs (gallium arsenide) FET is widely used.
In FIG. 1, the conventional amplifier 10 includes a DC-DC converter 11 and a HEMT GaAs FET Q1 (which will be hereinafter called the transistor Q1). The transistor Q1 has a source electrode which is grounded, and has a gate electrode which is coupled to a stripline L2 and a capacitor C3 in series. Hereinafter, the strip line will be referred to as a coil in the form fabricated on a printed circuit board. An input signal Vin, output from the plane antenna element of the antenna unit, is supplied to the gate electrode of the transistor Q1 via the capacitor C3 and the stripline L2. The transistor Q1 further includes a drain electrode which is coupled to two striplines L3 and L4 and a capacitor C4 in series. An output signal Vout is output from the drain electrode of the transistor Q1 via the striplines L3 and L4 and the capacitor C4.
In the conventional amplifier 10 in FIG. 1, the DC-DC converter 11 converts a positive source dc voltage +5 V, supplied from an external power supply (not shown) via a source power terminal 12, to a negative source dc voltage -5 V. In other words, the conventional amplifier 10 requires a first positive DC voltage supply and a DC-DC converter to supply a negative DC voltage. The amplifier 10 further includes a constant current circuit 13 and a voltage biasing circuit 14.
The constant current circuit 13 mentioned above includes: a resistor R3 connected to an output of the DC-DC converter 11; a resistor R10 connected to the source power terminal 12; a transistor Q2; a diode D2; a resistor R6 interposed between the resistor R3 and the transistor Q2; a resistor R7 interposed between the transistor Q2 and the resistor R10; a resistor R8 connected to the transistor Q2; and a resistor R9 interposed between the transistor Q2 and the diode D2. One end of the resistor R8 is grounded.
In the conventional amplifier in FIG. 1, a connection point between the resistor R7 and the resistor R10 and a connection point between the stripline L3 and the stripline L4 are connected by a stripline L5. The current through the drain electrode of the transistor Q1 is biased by the constant current circuit 13 through the striplines L3 and L5.
The voltage biasing circuit 14 mentioned above includes the resistor R3, a resistor R4, a stripline L1, a resistor R5, these three circuit elements being connected to the resistor R3 in series, and a stripline L1 L2 interposed between the resistor R5 and the transistor Q1.
In the conventional amplifier described above, the difference between the positive source dc voltage (+5 V) and the negative source dc voltage (-5 V) is divided by the resistors R10 and R7, the transistor Q2 and the resistors R6 and R3, and the connection point between the resistor R3 and the resistor R6 is set to a negative potential with respect to the ground level.
The negative potential at the connection point between the resistors R3 and R6 is applied to the gate electrode of the transistor Q1 through the resistor R4, the stripline L1, the resistor R5 and the stripline L2. Thus, in the conventional amplifier 10, it is necessary that the gate electrode of the transistor Q1 (HEMT GaAs FET) is biased at a negative potential, the source electrode is grounded, and the transistor Q1 amplifies the input signal under this condition to output the amplified input signal from the drain electrode.
Therefore, the conventional amplifier described above requires using a DC-DC converter, which is expensive, for the conversion of the positive source dc voltage (e.g. +5 V) to the negative source dc voltage (e.g. -5 V). For this reason, the conventional amplifier has a problem in that it has a complicated structure, and packaging the amplifier on a printed circuit board with a small size and high packaging density is difficult. Also, the cost of manufacture of the conventional amplifier is high because of the use of a DC-DC converter.